A worldwide telecommunication revolution is in progress. This revolution is independent of transmission media, information nature, and transmission speed. A unified means of moving information is a necessity for Asynchronous Transfer Mode (ATM), Internet, and satellites. However, problems exist when such unification is attempted.
ATM has been envisioned as a universal basic multimedia network architecture, which includes Internet and satellites. Multimedia consists of a set of media attributes on a set of multiple values. The media attributes are: compression, computation, distribution, generation, management, presentation, processing, retrieval, storage and transmission. The multiple values include audio, data, graphics, image, text, voice and video.
National and international standards have been established separately. These standards apply not only to commercial and civil telecommunications, but also to military telecommunications. These standards are used not only in satellite and space channels, but also in terrestrial microwave and fiber optical channels. These standards apply not only for fixed stations, but for mobile and personal communications also. The backbones to all multimedia transmissions are the protocols of ATM, Internet, and satellite communications.
Other error correction schemes for ATM networks have been proposed and used for testing in connection with ATM. Previous works have emphasized the nature of service type, loss compensation due to message over flow, specific encoder/decoder evaluation. Previous work also disregards the requirement of single cell transmission and/or de-emphasizes the functionality structure. All practical forward error correction schemes apply at the Physical Layer. The ATM information is treated as any stream of binary digits. The function of ATM cell structure is disregarded. As a consequence, the purpose and the foundation of ATM are ignored in currently available systems.
A subcommittee was formed within the ATM Forum Technical Committee to look into the performance aspect of ATM. In particular, five organizations have jointly contributed a document entitled “Necessity of an FEC Scheme for ATM Networks.” The contribution discusses the benefits of and the need for an AAL-level forward error correction (FEC) scheme in ATM networks.
In the Technical Reference TR-NWT-001112, issued by Bell Communications Research, the code polynomial used to specify the ATM Header Error Check is: H(x)=x8+x2+x+1. In terms of error correction, this code can only correct a single error within 32 bits of cell header information. With the above code an ATM error-coding test was performed through a satellite channel. AT&T in the U.S., KDD in Japan, and Telestra in Australia jointly conducted the test. The above Header codec, or a modified version was used in the transmission link. During the tests, a significant problem was observed: when the satellite channel encounters more than a single error in the cell Header, the Header codec produced additional errors in the decoder output. This undesirable characteristic of the HEC led the testing team to turn off and by-pass the Header codec.
There is no sufficient current technology in combinatorial set generation, in the creation of multimedia unequal weighted convolutional codes, multimedia coding strategy, the method of conversion all convolutional codes to block codes without degrading any error correcting capability, the domino effect of significant Internet improvement, establishing a method of evaluation for combining efficiency, reliability, and delay in ATM cell collision control mechanism for random multiple access satellite communications, and in protocol standard compatibility within ATM, Internet, and satellites.
Separatability, efficiency, reliability, and protocol problems exist in ATM, Internet, and satellite communications. Additional problems arise when the three disciplines are combined. The present state of efficiency and reliability are poor, and protocols are generally incompatible. Reliability is defined in terms of either burst error statistics or random error rate. Efficiency is defined in terms of throughput and delay, which in turn reflect the state of congestion and channel utilization. The significant limitations and problems related to the fields of inventions are outlined below.
The lack of applicable combinatorial sets—Previous work has linked optimal solutions in telecommunication with combinatorial difference sets. Unfortunately, two problems exist: for applications such as ATM code derivation and random multiple access sequence generation, the known methods of sets, which derived from projective geometry, cannot be applied. The number of sets derivable from projective geometry is severely limited for most applications.
Network and throughput problems—Even with a network of multiple spacecraft, multiple antenna beam hopping, and onboard processing, at any orbiting level, satellites with ATM and Internet are fundamentally not compatible, because satellite networks cannot provide the originally conceived ATM and Internet environment, multiple path fast packet switching. Even with ATM and Internet switches, routers, and a switching center installed onboard, satellites cannot obtain the flexibility envisioned by ATM and Internet. Existing or planned satellite systems cannot handle as large a number of users as the present and future Internet with ATM protocols. The existing signaling formats in either ATM or Internet and those used in satellite are not compatible. The delays in multiple hopped satellite links are not acceptable to ATM, and the Internet world. The error detection scheme used in the Internet is a very inefficient and costly way to use satellite channel. The present speed of Internet is slow in comparison to wideband satellite transponders. In addition, ATM, Internet protocols are not compatible with satellite transmission protocols.
Protocol and reliability problems—There is no provision for retransmission in IP. That is why, even in the absence of heavy user traffic, different logon times have been experienced. This is due to either noise or interference in the transmission channel, and the IP is continuously discarding the Header when errors occur in IP. There are three types of ARQ: Stop and Wait, Go Back-N, and Select Repeat. The codes used in all ARQ schemes have been referred to as CRC (Cyclic Redundancy Code). The code generator polynomials degrees from 4, 7, 8, 12, 16, 24, and 32 have been standardized by the International Telecommunications Union—Technical (ITU-T). TCP assigns sequence numbers to each transmitted data block, and expects a positive acknowledgment from the Transport Layer from a receiver. If an acknowledgment is not received in a timely manner, the entire block is retransmitted. This retransmission protocol is a variation of the Select Repeat—ARQ. When a transmission channel is noisy, all transmission schemes based on ARQ become inefficient.
Present Internet, based on the protocols of TCP/IP, has not been able to provide high reliability and efficiency. If a feedback channel is available, repeated transmission decreases efficiency. If a feedback channel is not available, one takes a big chance at the receiving end. Furthermore, for some transmissions in real time when errors occur, it is not always feasible to stop in the middle of a transmission and request for retransmission over a satellite link.
Delay and speed problems—Error detection and retransmission has been introduced in ATM, Internet, and satellites. When a transmission channel is either noisy or interference prone, repeated transmission increases, which causes either excessive delay, or the channel breaks down due to excessive re-transmission. Thus error detection and retransmission schemes are undesirable in a noisy environment such as wireless.
In satellite communications, particularly with Geosynchronous orbiting satellites, the round trip path delay is a well-known troublesome phenomenon. That is why echo canceller was developed for speech over the satellite link. The present transmission speed of the Internet is too slow to be efficiently using satellite transponder capacities. The Network Working Group established by the Internet Society, which is related to IETF, addresses the issue of enhancing TCP over satellite channels. The Group consists of organizations like NASA/Glenn Research Center, Nortel, USC, UC Berkeley, JPL, and Ohio University. IP Multicast Initiative has looked into TCP/IP issues with respect to satellites. TCP Over Satellite (TCPSAT) has been chartered, and the Internet Protocol over Satellite (IPoS) Working Group has been formed. The performance optimization of Internet protocols via satellite has been supported by European Space Agency and studied at the University of Aberdeen. These and other references indicate that the problems exist and have been identified. A large amount of effort has been devoted and directed to window size scaling, slow-start modification, time stamping, fast retransmit and recovery, and selective acknowledgement. Some TCP performance enhancement have been observed, but the basic structure of error mechanism in TCP/IP is untouched. Thus far, no organization has provided or suggested any concrete fundamental solutions.
Limitations and problems of known sequences—Almost all CDMA networks are implemented with linear feedback shift registers (LFSR) with modulo-2 adders over the binary field, GF (2). For any m stage LFSR the number of such generators, which can generate a maximum binary sequence length of 2m−1=n, is N=E(n)/m. Where E(n) is the Euler Function of n, or the number of integers less than n that are relatively prime to m. Thus for a given n, the number of LFSR stages, the number of direct sequences (DS), gold sequences (GS), or pseudo noise (PN) sequences generators is a direct function of the Euler Function.
Unfortunately E(n) is limited for a very small number of odd m. For a large value of E(n), the period or length of the sequence becomes long and thus inefficient. On the other hand, for most CDMA applications, the ability of accommodating a large number of users is a must. The number of users is directly related to the number of sequence generators. In some designs this relationship is one to one. As an example, for m=15, the sequence length is 32,767 and there exist only 42 generators.
By multiplication of two equal m degree minimal polynomials of related roots, the cross-correlation of a pair of sequences generated by the two minimal polynomials are known and bounded. Such sequences, referred as Gold Code or Gold Sequences (GS), have been suggested for spread spectrum applications. GS has 2m+1 different number of sequences and the period of the sequences are 2m−1. Although the correlation property of the sequences is known, the distance property among the sequences is not.
For DS, PN, or GS the limitations for the desired applications are failures to provide a large number of sequence generators, and the inability to shorten the sequence length in the detection processes. When designing for the military, such inefficiency may be tolerable and justifiable for extremely high reliability. It is neither cost effective, nor efficient when a system or a product is designed to compete in the commercial world market. For multi-user transmission, synchronization needs to be established frequently. The long length of DS or PN makes the system operation very inefficient. Separate arrangements for synchronization may be used, with DS or PN used for messages only, but such an arrangement is a waste from a system optimization viewpoint. The desired solution is a sequence, which can be used for synchronization, user identification, and secured message carrying all at the same time.
Problems of existing non-binary sequences—Normally, powerful non-binary code words such as Reed Solomon (RS) should be ideal for CDMA, FH, and/or non-Aloha random multiple access (RMA) applications. In fact, prior to this invention, RS code words were the best solution. The problems of RS code words are: a. For the same number of symbols, the RS code words are orders of magnitude longer than the sequence obtainable by the method of this invention. b. Few extremely low rate RS codes exist.
Limitation of orthogonal signaling—Orthogonal signaling can eliminate the interference problem, but the signaling set is too small to be usable for large number of accessing users. Once the distance property of the sequences is defined this way, all the existing error correcting code words based on either Hamming or Lee distance become useless in such applications.
The confinement and limitation of error coding in ATM—For the ATM header, it is a single error correction block code. For this code, when a transmission channel encounters more than a single error in the cell header, the header decoder produces additional errors at the decoder output. The result is that the error corrector becomes an error generator. This phenomenon exhibits a very undesirable characteristic in any information transmission. For the scheme of two dimensional encoding proposed by the ATM Forum subcommittee, the delay in decoding is excessive and the format does not match with the exact number digits in the information field of an ATM cell. In addition, there is no safe guard for a loss cell recovery mechanism. When error coding is confined within the ATM cell switching structure, it becomes a very difficult task.
The ATM Forum Recommendation identifies the difficulty of error coding in ATM. The recommended two-dimensional coding scheme can only protect the information part of the ATM cell, but not its header. The consequences can be that high fidelity messages may either end at a wrong destination, or no destination at all. Improving information quality on cell basis is important, but more important is the ability to transport a cell reliably regardless the quality of a message. But this is also a difficult task, and the ATM Forum has not addressed any solution.
The Limitation of Block Codes—All protocols in ATM, Internet, and satellite communications are block structured. Standards make all the protocol structures rigid. Few block codes can meet the strict requirements of the standards particular at short code block lengths. Because the error correcting capability of block codes is proportional to the code block lengths. On the other hand, convolutional codes are more powerful in terms of error correcting capability even at very short code length. Unfortunately, convolutional codes rely on memory to encode and decode. That is, convolutional codes cannot have the independent block structure as required by all the protocols.
The “impossibility” of construct block codes from convolutional codes—Block codes are known to encode and decode with independent code words with block structure. Convolutional codes are known to encode and decode with continuing dependent coded digits. That is, memory exists and is required in convolutional encoding and decoding. At shorter length, convolutional codes have more error correction capability. But, all protocols are block structured and few block codes can solve the coding problems particular at short length protocols. From theory to practice, it is “impossible” to optimally convert a convolutional code to a block code.
The non-existence of multimedia unequally weighted convolutional codes—In multimedia, the significance of the values of the media attributes is different. The efficiency and reliability requirements are different for voice, video, and data transmission. To conserve bandwidth, source coding is applied and data compression is used. The consequence of either source coding or data compression is that some digits are more significant than others. If error coding is applied indiscriminately, it is inefficient, unreliable, or wasteful. In multimedia, the most significant digits need to be highly error protected, and the least significant digits are sufficient to be modestly error protected. A few unequal error protection block codes are available, but their use is limited. For the large class of powerful convolutional codes, the method of producing such code with unequal error protection is not available.
The problems of standards and their compatibility—In the telecommunications industry, standards dominate product acceptability. If a new scheme does not meet the standards, the scheme at best remains as a patent. At present, the protocol standards for ATM, Internet, and satellite communications are all different and not compatible.
Wireless Problems—Wireless transmission channels are characterized with more errors and interference than transmission with wires. Wireless standards come from IEEE 802.11 and are limited to physical layer and the common medium access control of wireless LAN. For wireless Internet, the Internet Engineering Task Force (IETF) is working to establish standards for mobile internetworking through Internet Protocol version 6 (IPv6). For satellite communications, both ITU-T and ITU-R issue international standards for both “fixed” and mobile satellite services. For international digital satellite communications, the series of INTELSAT'S SSOG (Satellite Systems Operations Guides) are the industry's standards.
Modulation Standard Problem—The ITU-T latest standards for modulation is V.90, which is based on V.34, which in turn depends on V.42 standard for “error correcting” procedures. V.42 is misleading because the procedure does not correct any errors. The procedure is a basic error detection and retransmission scheme. The effective error correction may be achieved only after many retransmissions.
QoS Problems—For general optimization of system reliability, Tillman, Hwang, and Kuo provide a fundamental reference. Roosta addresses routing through a network with maximum reliability. Przygienda addresses the subject of Link State Routing with QoS in ATM LANs, in which, the optimization of ATM paths is taken into account the various QoS requirement, such as bandwidth and/or bit error rate. Computation of paths satisfying all QoS conditions has been identified as NP-complete and thus computational non-tractable. Further work is required in order to pre-compute the paths with guaranteed quality of services. The issues of very large number user networks and reliable routing capabilities are yet to be answered.
Needs exist for improved methods of transmission for ATM, Internet and satellite communications.